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Renewable and Sustainable Energy Reviews 16 (2012) 1061– 1073

Contents lists available at SciVerse ScienceDirect

Renewable and Sustainable Energy Reviews

j ourna l h o mepage: www.elsev ier .com/ locate / rser

olar drying of wastewater sludge: A review

yes Bennamoun ∗

aboratoire des Composants Actifs et Matériaux, Faculté des Sciences Exactes, Sciences Naturelles et de la Vie, Université Larbi BenM’hidi, Oum El Bouaghi, Algeria

r t i c l e i n f o

rticle history:eceived 16 September 2011ccepted 12 October 2011vailable online 4 November 2011

eywords:olar energyrying kinetichrinkageracksesignreenhouse

a b s t r a c t

Drying constitutes an important process for wastewater sludge management, as it can reduce the massand the volume of the product and consequently the cost of storage, handling and transport. During con-stant operating conditions, the drying kinetic of the sludge has shown mainly: a constant drying rate,one or two falling rate periods and a final short period with variations along the process of the physicalproperties of the product with the appearance of shrinkage and cracks phenomena. Solar drying wasbenefit as using free solar energy can reduce the cost of the operation. On the other hand, it plays animportant role for the pathogen reduction until Environmental protection Agency (EPA) recommenda-tions. In some studied cases, the value of 1000 CFU g−1 DS, which represents the EPA Class A pathogenrequirement, for fecal coliform was attained. The general design of used solar dryers was constituted of:a greenhouse made with transparent material and a floor, where the product is speared in thick layers.Furthermore, fans and ventilations can be used in order to have homogeneous distribution of the airinside the greenhouse with replacement of humidified air with fresh one. Automatic or handle mix of

the product was used once or for several times a day. In order to increase the performances of the dryingsystem, other ways such as heating the floor using solar water heater, infrared lamps, using heat pumpsor adding thermal energy storage systems were also tested. Covered solar drying has given better resultsthan open solar drying. However, the origin of the wastewater sludge affects the obtained results. Alter-natively, modeling drying systems was effectuated using heat and mass balances, applied for the air andthe dried product. Solar drying of wastewater sludge has given satisfactory results.

© 2011 Elsevier Ltd. All rights reserved.

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10612. Behaviour of wastewater sludge during drying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10633. Phenomena occurring to wastewater sludge during drying process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10644. Solar drying of wastewater sludge: a review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10665. Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1072

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1072

. Introduction

Water constitutes one of the keys of the socio-economicevelopment and the human daily comfort. Consequently, theroduction of water increases with the population increase. Statis-

20 l per day in some regions such as Mali or Haiti [1]. In general man-ner, statistics show that 70% of water produced quantity is used foragriculture especially for the irrigation; however 30% are directedto the industry [1]. These important consumed quantities of waterengender, of course, the production of wastes that, in some cases,

ics confirm the non-homogeneous distribution of this importantesource around the world; as the consumption of an inhabitant ofurope is estimated to 3000 m3 per year. This quantity increaseso 9985 m3 per year for an inhabitant of the U.S.A. and decreaseso 200 m3 per year in the developing countries and attains

∗ Corresponding author. Tel.: +213 32 42 39 83; fax: +213 32 42 39 83.E-mail address: lyes bennamoun@yahoo.ca

364-0321/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.rser.2011.10.005

can be noxious to the environment. The use of wastewater treat-ment plants (WWTP) was one of the novel developed solutions thatpermit to have drinkable water from wastewater; nevertheless itcauses the production of sludge. In 2009, over 9.18 million tons ofdry solids sludge is produced in China from WWTP [2]. This quan-

tity was 5.36 million tons in the U.S.A. and around 2.00 milliontons in Japan, in 1996 and 1993 respectively, as reported by Chenet al. [3,4]. The European commission [5] has given detailed statis-tics about the total production, in the last decade, of sewage sludge

1062 L. Bennamoun / Renewable and Sustainable

Nomenclature

A area (m2)a solar absorption coefficientCp specific heat (J kg−1 ◦C−1)E evaporation rate (kg of water m−2 of ground h−1)h convective exchange coefficient (W m−2 ◦C−1)Lv latent heat of vaporization (J kg−1)M mass (kg)P solar radiation or power (W m−2)R radiative heat (W)T temperature (◦C)t or � time (s)V volume (m3)W or X moisture content (% or kg kg−1 dry basis)w humidity ratio (kg of water kg−1 of air)

Greek symbols� density (kg m−3)� coefficient of transmission

Subscriptsa airBo sludgeext externalin air inletout air outlets solidV sup glass roofV glass0 initial value

Table 1Total sewage sludge production for urban wastewater (values presented by millions of ki

2000 2001 2002 2003 2

Belgium 101 112 118 109

Bulgaria 48 45 40 43

Czech Rep. 207 206 211 180

Germany 2429 2Ireland 34 38

Greece 68 78 80

Spain 853 892 987 1012 1France 954 1Hungary 102 115 117 152

Netherlands 346 358 365 353

Austria 315 323

Poland 360 397 436 447

United Kingdom 1527 1544 1656 1

Source: [4].

Table 2Agriculture use of sewage sludge from urban wastewater (in % of the total value).

2000 2001 2002 2003 2004

Belgium 9.82 9.32 17.43 18.10

Bulgaria 0.00 0.00 0.00 0.00 0.00

Czech Rep. 74.76 75.36 10.00 17.32

Germany 31.08 27.78

Ireland 41.18 44.74

Greece 0.00 0.00 0.00 0.00

Spain 53.22 67.94 66.67 66.21 65.11

France 50.42 43.87

Hungary 26.47 22.61 25.64 19.08 22.28

Netherlands 0.00 0.00 0.00 0.00 0.00

Austria 11.75 12.07 12.46

Poland 14.17 12.34 15.37 12.98 14.08

United Kingdom 55.66 54.60 64.01 64.96

Source: [4].

Energy Reviews 16 (2012) 1061– 1073

from urban wastewater (Table 1). It shows that some countries haveincreased considerably their produced quantities such as Greece,Hungary, Spain, France, Poland and the United Kingdom, converselyother countries have kept a constant production or decreased it,similar to Belgium, Bulgaria, Czech republic, Germany, Netherlandsand Austria. Nowadays, agriculture, incineration and landfill usesconstitute the major applications of sludge disposal. The Europeancommission [5] has given more details for the most relevant pro-ducer countries. Table 2 shows that Spain, United Kingdom andIreland are widely using more than 50% of their sewage sludgeobtained from wastewater in agriculture against France, Germany,Hungary and Czech Republic which present moderate use between28 and 47%. Until 2009, Greece and Netherlands have never incor-porated their sewage sludge in agriculture. Netherlands, Austria,Belgium and Germany prefer the incineration use of their sludgewith rates equal to: 73.62%, 44.31%, 41.01% and 36.20% respectively.France and United Kingdom present small values which did notexceed 18%. However, Bulgaria, Czech Republic and Hungary didnot use incineration with a rate near zero. Greece uses the landfillfor almost the totality of their sewage sludge with a rate around86% as well as a rate of 100% from 2001 to 2005. The rate is more orless the third for Bulgaria, Ireland, Hungary and Poland. Accordingto the values given by Chen et al. [3,4], U.S.A. is using 33.3% of theirglobal sludge production in agriculture, 16.1% in incineration and34% in landfill. However, for Japan these values are 0%, 75% and 25%respectively (Tables 3 and 4).

In all cases, drying constitutes an essential process, after appli-cation of the mechanical dewatering by centrifugation or filtration.

It can reduce the water content below 5% dry solids leading to thedecrease of waste mass and volume and in consequence the reduc-tion of the cost for storage, handling and transport. On the otherhand, drying sludge increases the low calorific value, transforming

lograms).

004 2005 2006 2007 2008 2009

116 113 128 129 14058 42 38 40 43 39

179 172 175 172 220261 2170 2049

60 8883 117 126 134 136 152

092 1121 1065 1153 1156 1205060 1087184 260354 359 373 353 353305 255 254476 486 501 533 567 563721 1771 1809 1825 1814 1761

2005 2006 2007 2008 2009 Mean value

18.58 14.06 7.75 7.86 12.870.00 31.58 15.00 25.58 35.90 10.81

16.86 19.43 27.91 25.00 33.3341.98 29.87 32.6876.67 54.19

0.00 0.00 0.00 0.00 0.00 0.0064.76 64.51 74.93 80.19 82.57 68.61

47.10 47.1356.92 28.83

0.00 0.00 0.00 0.00 0.00 0.0015.29 15.75 13.46

13.58 16.17 18.39 19.75 21.85 15.8768.94 61.64

L. Bennamoun / Renewable and Sustainable Energy Reviews 16 (2012) 1061– 1073 1063

Table 3Incineration use of sewage sludge from urban wastewater (in % of the total value).

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 Mean value

Belgium 58.93 60.17 17.43 24.14 31.86 33.59 52.71 49.29 41.01Bulgaria 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Czech Rep. 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.36 0.17Germany 22.85 31.45 43.41 47.10 36.20Ireland 0.00 0.00 0.00 0.00Greece 0.00 0.00 0.00 0.00 0.00 0.00 1.49 17.65 0.00 2.13Spain 8.21 6.17 6.99 7.61 7.05 6.96 3.85 0.00 0.00 5.20France 17.40 16.79 18.95 17.71Hungary 0.00 0.00 0.00 0.00 0.00 0.77 0.13Netherlands 52.02 58.10 55.89 63.46 75.99 82.73 87.13 92.07 95.18 73.62Austria 47.62 50.15 49.51 38.43 35.83 44.31Poland 1.67 1.76 1.61 1.34 0.21 1.23 0.80 0.38 1.06 1.60 1.17United Kingdom 15.78 19.82 18.96 15.86 15.92 17.27

Source: [4].

Table 4Landfill use of sewage sludge from urban wastewater (in % of the total value).

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 Mean value

Belgium 16.07 11.02 13.76 7.76 3.54 0.00 0.00 0.00 6.52Bulgaria 0.00 0.00 0.00 0.00 87.93 54.76 42.11 52.50 41.86 28.21 30.74Czech Rep. 18.45 18.96 12.78 13.97 6.98 8.00 5.23 12.27 12.08Germany 6.59 3.49 1.24 0.24 2.89Ireland 50.00 52.63 16.67 39.77Greece 100.00 100.00 100.00 100.00 100.00 97.62 55.22 52.94 71.71 86.39Spain 17.94 14.69 16.31 16.11 14.93 14.54 15.77 0.00 0.00 12.25France 24.11 20.94 8.28 17.78Hungary 46.08 47.83 40.17 40.13 28.80 29.62 38.77Netherlands 18.50 17.60 10.96 4.53 4.24 3.90 4.02 0.00 0.00 7.08Austria 13.33 11.46 9.84 9.80 8.27 10.54Poland 42.22 50.13 44.04 36.91 34.24 31.07 29.34 23.45 16.23 14.56 32.22

.89

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thm

hretoc

2

peccatCavrlotdtmTs

superficial velocity, lead to the increase of the drying rate. On theother hand, we observe at the beginning of the process (at the pointX = 2.6 kg kg−1 to around 2.4 kg kg−1) the existence of a very shortfirst period caused by the adaptation of the product to the operating

United Kingdom 7.99 8.03 5.31 3

ource: [4].

he product into an acceptable combustible. Also, the treatment byigh temperatures makes the product as pathogen free stabilizedaterial [6–8].The physico-chemical composition of several sewage sludges

arvested from different WWTP in the world and the elementsange are represented in Table 5 [3,4,9–11]. The results show thexistence of important minerals that can be functional for agricul-ure and soil utilization and other elements that must be treatedr eliminated. Moreover, the table shows difference in the elementoncentrations, depending on the source of the sludges.

. Behaviour of wastewater sludge during drying

The general behaviour of the wastewater sludge during dryingrocess, under constant operating conditions is divided by Dengt al. [2], and confirmed by Vaxelaire and Cézac [12], into 4 prin-ipal parts, as shown in Fig. 1, which represents a typical dryingurve. The water removed in the first part (line AB) is considereds free water. After that, we have a succession of two falling rates;he first is represented by the line BC and the second by the curveD. The water removed in these parts is, respectively, interstitialnd surface water. The last period of the curve is a short inter-al, where bound water is removed and equilibrium moisture iseached. However, Deng et al. [1] observe in their experimentalabour the inexistence of the constant drying rate during dryingf municipal sewage sludge. They report this observation due tohe utilization, before applying drying process, of the mechanicalewatering which removes all the free water. The same observa-

ion was made by Tao et al. [13] during their utilization of the

echanical dewatering of different types of wastewater sludges.he experimental work developed by Léonard et al. [7,14] hashown that the behaviour of the wastewater sludge during drying

5.42 6.13

depends on the origin of the sludge, as illustrated in Fig. 2. SludgeA shows a clear constant drying rate period which is not the caseof sludge B. However, the other falling periods and the short finalperiod are observed for both sludges. Reyes et al. [15] and Léonardet al. [14] have studied the effect of the drying operating conditions,in particular the influence of the air temperature and its superficialvelocity, as exposed in Fig. 3(a) and (b). The results presented inthese figures point towards that drying are strongly affected bythe air temperature and with a smaller degree by its superficialvelocity, similar results were found for the treatment of food dry-ing [16–19]. In general way, increasing, the temperature and the

Fig. 1. Typical sludge drying curve [2].

1064 L. Bennamoun / Renewable and Sustainable Energy Reviews 16 (2012) 1061– 1073

Table 5Physico-chemical properties of several sludges.

Parameter Sludge A Sludge B Sludge C Range

pH 7.6 ± 0.8 7.6 ± 0.1 5.0–8.0Moisture content (%) 85.0Dry matter (%) 18.0 ± 0.6Total solids (TS, %) 20.6 ± 1.8 3.0–7.0Volatile solids (%) 60.4 ± 2.1 (of TS) 72.9 (of dry solids DS) 60–80 (of TS)Nitrogen (N, %) 5.38 ± 2.13 3.34 ± 0.1 1.5–5.0 (% of TS)Phosphorus (P2O5, %) 2.7 ± 0.6 2710 ± 9.2 mg kg−1 0.5–2.8 (% of TS)Potash (K2O, %) 0.0–1.0 (% of TS)Heat value (kJ kg−1 dry basis DB) 18,500–30,000Alkalinity (mg l−1 as CaCO3) 500–1500Arsenic (As, mg kg−1 DB) 44.9 ± 5.7 1.1–230Cadmium (Cd, mg kg−1) 1.3 ± 0.4 1.5 (DS) 2.0 ± 0.5 1–3410 (DB)Chromium (Cr, mg kg−1) 321 ± 15 26.7 (DS) 243 ± 10 10–99,000 (DB)Copper (Cu, mg kg−1) 388 ± 18 154.4 (DS) 19 ± 1.0 84–17,000 (DB)Lead (Pb, mg kg−1) 29.2 ± 3.6 43.3 (DS) 34 ± 2.0 13–26,000 (DB)Mercury (Hg, mg kg−1 DB) 0.6–56Molybdenum (Mo, mg kg−1 DB) 0.1–214Nickel (Ni, mg kg−1) 128 ± 12 21 (DS) 79.0 ± 3.0 2–5300 (DB)Selenium (Se, mg kg−1 DB) 1.7–17.2Zinc (Zn, mg kg−1) 541 ± 73 616.3 (DS) 1435 ± 15 101–49,000 (DB)Iron (Fe, mg kg−1) 10,375 ± 675 5128.6 (DS) 1000–154,000 (DB)Cobalt (Co, mg kg−1 DB) 11.3–2490Tin (Sn, mg kg−1 DB) 2.6–329

144.

S

ca[wbwdwci

3d

eps

ot

F

in food drying [18,27–29]. In addition, shrinkage was observedduring wastewater sludge drying by Tao et al. [13] (Fig. 5). Theupper images of the figure were scanned using micro-computerizedtomography scanner (micro-CT) and the lower images are the 2-D

Manganese (Mn, mg kg−1) 165 ± 8

ource: Sludge A: [9], Sludge B: [10], Sludge C: [11] and Range: [3,4].

onditions and represented by an increase of the drying rate. Usu-lly, this period is neglected in modeling and simulation works15,20,21]. Léonard et al. [6] have experimented in their work twoastewater sludges. The constant drying rate, made known in Fig. 4

y “reference”, was clearly observed. One of the objectives of theork was to study the effect of the expansion of the sludge extru-ates bed due to increasing additions of dry product and variationsere followed using X-ray tomography. As shown in the figure, the

onstant drying period was made shorter with the bed expansionncrease until it roughly disappearance.

. Phenomena occurring to wastewater sludge duringrying process

Léonard et al. [8,22,23], Tao et al. [13], Hsu et al. [24] and Ruizt al. [25,26] observed the creation of two phenomena that takelace inside the sludge during drying process. The phenomena are:

hrinkage and cracks.

Shrinkage consists on the reduction of the product size becausef the loss of its water during drying process. In spite of the impor-ance of the phenomenon and the engendered changes in the

ig. 2. Drying kinetic curve for two sludges at 160 ◦C, 1 m s−1 and 0.006 kg kg−1 [13].

4 (DS) 32–9870 (DB)

product physical properties, it was recently introduced especially

Fig. 3. Influence of the operating conditions on the behaviour of the drying kinetics[14]: (a) Va = 0.65 m s−1, (b) Va = 0.43 m s−1.

L. Bennamoun / Renewable and Sustainable Energy Reviews 16 (2012) 1061– 1073 1065

ter sl

rwoTo

Fig. 4. Drying kinetics of two wastewa

econstituted transversal section of the upper images. In the same

ay and as shown in Fig. 6, Léonard et al. [8,22,23] follow the effect

f the moisture content decrease using X-ray microtomography.he water content inside the product was calculated on the basef the variations of the grey level of water, wet and dry solid. On

Fig. 5. Observation of shrinkage during

udges with back mixing influence [5].

the other hand, the authors have observed cracks at the interior

and the surface of the dried product at the same time as shrinkagehappens. Shrinkage starts with the drying process begin (Fig. 7)and continue until reaching a constant value evaluated between0.4 and 0.2 of the initial volume, depending on the origin of the

wastewater sludge drying [13].

1066 L. Bennamoun / Renewable and Sustainable Energy Reviews 16 (2012) 1061– 1073

Fig. 6. 2D cross section images at different water content illustrating shrinkage and cracks development [23].

Table 6Initial and final dimensions of extrudate samples of two different wastewater sludges [8].

Sludge A Sludge B

Extrudate 1 Extrudate 2 Extrudate 1 Extrudate 2

Initial equivalent diameter (mm) 12.74 ± 0.18 12.64 ± 0.08 13.00 ± 0.32 13.11 ± 0.12Final equivalent diameter (mm) 7.40 ± 0.44 7.48 ± 0.39 10.09 ± 0.23 9.35 ± 0.90

wt7gsvioo

Initial height (mm) 15.98 ± 0.34

Final height (mm) 9.29 ± 0.20

astewater sludge. Table 6 gives the initial and final dimensions ofhe dried product. It shows an isotropic reduction varying between7% and 58%, in both diameter and height of the product, whichives the announced precedent shrinkage rates. However, crackshow constant low values at the beginning of the process. Thesealues increase with the moisture content decrease, until attain-

ng values around 0.35 and more than 0.5, with an influence of theperating drying conditions, as illustrated in Fig. 8 and the originf the sludge.

Fig. 7. Shrinkage curve for two different wastewater sludges [8].

15.91 ± 0.33 16.29 ± 0.29 16.36 ± 0.289.04 ± 0.40 12.67 ± 0.10 12.67 ± 0.07

4. Solar drying of wastewater sludge: a review

Applying free solar energy may well be an alternative solutionfor reduction of the cost of the drying process. However, contraryto drying with constant conditions, the obtained drying kineticsduring solar drying present some variations, as the operating condi-tions continuously change with time. It was found that at variationof the operating conditions; the product adapts its comportment tothe new conditions by varying the common direction of its dryingkinetic with a non-instant adaptation and registration of a time ofreaction [18,30,31].

Salihoglu et al. [9] present the performances of a solar dryingplant situated in Bursa city at the southeast of the Marmara Seain Turkey. By the end of 2006, this country has produced around27,000 tons of dry solids per year. Wastewater sludge was obtainedfrom a municipal plant which has a capacity of 64,000 m3 perday. Covered and open sludge drying plants were constructed withfloor dimensions of 2 m × 5 m to conduct the experiments. The twoplants were operated according to the conceptual model shown inFig. 9. For both plants, the sludge was spread out on the floors in25 cm-layers.

The covered sludge drying plant was constructed as a tunneltype greenhouse with a roof height of 2.5 m. It was completelyenclosed by two walls, 10 mm thick transparent polycarbonatesheet with light transmittance of 80%. The indoor air accumulatedin the plant during the day is directed to the rock-bed for energystorage. The schematic view of the plant is shown in Fig. 10. Aera-tion of the surface of the sludge is effectuated by a ventilator fixed

in the roof. The saturated sludge surface layer was removed withthe ventilator turbulent air. Besides, saturated air by accumulationwas discharged by means of two fans mounted above the doors ofthe plant, and air renewal was provided. The impermeable concrete

L. Bennamoun / Renewable and Sustainable Energy Reviews 16 (2012) 1061– 1073 1067

ring d

fltbuwlm

fwsd

Fig. 8. Development of cracks du

oor of the greenhouse was heated using hot water pipes connectedo two flat plate solar collectors. Under the concrete floor, a rock-ed, composed of 16–48 mm diameter stones and 50 cm depth, issed as storage system. The bottom and sidewalls of the rock-bedere insulated with an impermeable concrete floor and heat insu-

ation material. Every day and for two times, the sludge was mixedanually for sludge renewal purposes.On the other hand, an impermeable concrete floor is constructed

or the open sludge drying plant. Surface aeration was suppliedith the wind effect and sludge was heated with direct expo-

ure to the sun. Also, the sludge was mixed manually twice aay.

Fig. 9. Conceptual model for the

rying of wastewater sludge [23].

As commented before regarding the adaptation of the dry-ing kinetic its behaviour during application of variable conditions,Fig. 11 show the total dependence of the moisture content vari-ations on the climatic conditions with a non-regular decrease, inparticular sludge dried in the open plant. In this plant and after 55days of drying time the moisture content was around 60%. How-ever, it has reached 20%, for the same time, for a sludge dried in thecovered plant. With lime added the performances of both plants

were ameliorated and the total amount of the sludge to be disposedwould be reduced by approximately 40%. The financial evaluationof the covered drying system gave a self amortization in only 4years.

experimental design [9].

1068 L. Bennamoun / Renewable and Sustainable Energy Reviews 16 (2012) 1061– 1073

Fig. 10. Schematic view of the covered solar drying plant [9].

Fig. 11. Covered and open sun wastewater sludge drying in summer period [9].

nable Energy Reviews 16 (2012) 1061– 1073 1069

oodmsdiTCpidw

FprttS1ifsta0

L. Bennamoun / Renewable and Sustai

Concerning the pathogen removal, a greater reduction wasbtained within the covered sludge drying system comparing to thepen system. Fig. 11 shows the fecal coliform reduction obtaineduring the solar drying in the cover and open plants, in sum-er period. The coliform content of the mechanically dewatered

ludge was 107 before starting drying process. At the end of 45ays and at the application of the covered solar drying, the col-

form content decreased to below 2 × 106 CFU g−1 DS in summer.his is the limit value of the Environmental Protection Agency (EPA)lass B pathogen requirement. However, to obtain the EPA class Aathogen requirement of 1000 CFU g−1 DS in a shorter time; a lim-

ted amount of lime was added to the dewatered sludge before solarrying. An average time of 10 days in summer and 20 days in winterere needed.

Lei et al. [32] developed a laboratory greenhouse, shown inig. 12. The greenhouse is made of glass. Its inner bottom side isainted in black in order to increase absorption of solar energyadiations. A chimney of 180 cm high was placed on the top ofhe greenhouse. Also, 6 air vents were made on the two walls ofhe greenhouse. The drying system was tested in the region ofhanghai in china. It is characterized by a maximum irradiance of002 W m−2 in summer. It decreases to 947 W m−2 in autumn and

n spring and it decreases until 910 W m−2 in winter. Samples ofresh sludge were obtained from WWTP situated in Shanghai. The

ludge has initial moisture of 5.16 kg kg−1 dry basis and was driedo 0.78 kg kg−1 dry basis. It was putted in 25 mm thick layers in

plate which was made of 0.1 mm steel mesh and had an area of.22 m × 0.36 m. The drying process has taken 125 h in summer and

Fig. 13. Variations of the moisture content vs. tim

Fig. 12. Greenhouse used for sludge drying [32].

around 550 h in winter with a non-regular decrease of the moisturecontent, as illustrated in Fig. 13(a) and (b).

Mathioudakis et al. [10] propose drying wastewater sludgeusing two solar drying plants. The sludge was obtained from WWTP

e of the sludge for different seasons [32].

1070 L. Bennamoun / Renewable and Sustainable Energy Reviews 16 (2012) 1061– 1073

t) and

ottmwcoHppplaimcbche

sit2c8

uu

Fig. 14. Solar drying plants (lef

f Komotini in Greece. Several batches of sludge were sampled afterhickening and dewatering. The dewatered sludge was dried usingwo pilot-scale solar drying plants of approximate 2.5 m3 each and

ade of polycarbonate, as it is illustrated in Fig. 14. The first plantas equipped with gravel floor, where hot water, generated by a

ommercial solar water heater, circulated. It permits the utilizationf solar energy additionally to the energy given by the greenhouse.ot water circulation was controlled by a thermostat, and it waserformed only when the water temperature was higher than thelant indoor temperature. A fan was installed inside both plants torovide turbulent air able to remove the moisture from the surface

ayer. Also, two axial fans were replacing saturated air with freshmbient air. Approximately, 8 kg of dewatered sludge were placednto crates forming a depth between 20 and 25 cm. The sludge was

ixed manually once per day. The results show that the moistureontent of wastewater sludge has decreased from 85% to 6% wetasis within 7–12 days in summer and to 10% as final moistureontent within 9–33 days in autumn. Incorporating a solar watereater, with recirculation through the bottom of the plant has accel-rated drying process by 1–9 days during winter conditions.

Before drying, the total and fecal coliform of the dewateredludge was respectively 4 × 106 CFU g−1 DS and 3 × 105 CFU g−1 DSn summer and in autumn. After application of solar drying,he dry sludge has attained, in summer, the following values:

× 104 CFU g−1 DS as total coliform and 103 CFU g−1 DS as fecaloliform. These values were respectively 2 × 106 CFU g−1 DS and

× 105 CFU g−1 DS in autumn.Slim et al. [33] have modeled a wastewater sludge drying system

sing climatic conditions of the region of Agen in France. Sludge isniformly spread over a concrete floor of the greenhouse dryer. The

Fig. 15. Schematic diagram of the solar and

plant indoor view (right) [10].

drying system is composed of three main parts: the greenhouse,the sludge mixing engine and the heat pumps with correspondingheat sources and sinks (Fig. 15). The greenhouse is consists of ahall of 4.6 m width and 20 m length completely covered by a trans-parent shell, resistant to unfavourable climatic conditions. It limitsloosing and exchanging heat with external air. The roof of the green-house has a height of 4 m under gutter and 5 m at the top. Inside thegreenhouse, two walls of 0.45 m height are dividing the space of thehouse. The ventilation ducts are fixed on these walls to make a lat-eral sweeping of the house. Its floor is made of three layers: deepestone is composed of polystyrene to reduce the heat looses towardsthe ground. The second layer consists of a network of polyethy-lene tubes fixed to the polystyrene panels. The tubes are installedto ensure a uniform flux distribution inside the greenhouse. Thelast layer is made of 10 cm thick of concrete. Sludge is mixed insidethe greenhouse using a mixing engine that operate every 12 h andprevent the formation of crusts. Two heat pumps are used in thedrying system, one for heating the air and the second for heatingthe floor of the greenhouse. The heat pumps are controlled by anon–off strategy and its functioning depends on the climatic condi-tions. The authors have modeled and compared the efficiencies ofthe drying systems in different seasons. The modeling work wasbased on mass, energy and momentum laws as well as some heattransfer correlations. It was deduced that more detailed economi-cal calculations are needed to confirm the obtained results and thusthe use of the proposed model by the designers.

Roux et al. [34] tested and modeled solar drying of wastewa-ter sludge using a greenhouse situated in the region of Fonsorbesin France. The drying unit is made up of two drying bays and isabout 50 m long, 4 m height and 15 m wide. According to the design

heat pump sludge drying system [33].

L. Bennamoun / Renewable and Sustainable

gsctrFcAa

oo

if

�

f

�

Fig. 16. Transfer phenomena in the solar drying unit [34].

eometry, shown in Fig. 16, and the operating conditions, sludge ispread only in the first bay of the green house. The design of the unitontains windrows in order to offer an optimized transfer area withhe forced air flux. This high forced air renewal allows water evapo-ation from the sludge and the transport of the water out of the unit.ans were placed inside the unit for ensuring homogeneous wateroncentration and sufficient connective transfers with the sludge.lso, a robot regularly adds some fresh sludge within windrowsnd mixes them to homogenize dryness and temperature.

Modeling of the drying unit passes through the establishmentf heat and mass balances. So, two differential equations werebtained:

The boundary walls balances take into account solar effects,nternal and external convections and infrared radiations. The dif-erential equation of the roof of the greenhouse is written:

V supVV supCpV supdTV sup

dt= [PV supaV sup + hV sup(Ta − TV sup)

+ hext(Text − TV sup)]AV sup +∑

j walls /= V sup

Rj−V sup (1)

The second differential equation is written in the followingorm:

BoVBoCpBodTBo

dt= [PBoaBo�V + hBo(Ta − TBo)]ABo

+∑

j walls /= Bo

Rj−Bo + MsLvdX

dt(2)

Fig. 17. Comparison of drying rate curve for purely solar dry

Energy Reviews 16 (2012) 1061– 1073 1071

Experimental data are used for determination of the thin layerdrying of wastewater sludge.

The modeling work has successfully permitted to follow thevariations of the windrow height and the dryness evolution withtime. Also, it was possible to calculate the temperature inside thegreenhouse.

Krawczyk and Badyda [35] constructed and tested a pilotwastewater sludge dryer in Kamienna region in Poland. The mainpart of the dryer is a light steel structure covered with polycar-bonate plates built on a concrete plate. The structure was fittedwith ventilation system in order to provide a uniform distributionof the air over the surface of the dried sludge through a numberof blowers. Also, a sludge shuffling system was installed. The solardrying system has an average daily drying rate of 8.12 kg H2O/m2 d.In order to increase the performances of the drying system, an aux-iliary source is added. The first experienced auxiliary source wasinfrared (IR) lamps which gave a supplement of power assumedequal to 150 W m−2. The average daily drying rate has enhancedto 11.11 kg H2O/m2 d. The calculus of the usage efficiency gave arate of 57.7%. For the second investigation and in addition to solardrying, floor was heated. The average daily drying rate has attained11.71 kg H2O/m2 d, with an average power output of the floor heat-ing system equal to 225 W m−2 and the usage efficiency were 46.2%.The variations of the drying rate for the three proposed methods areshown in Fig. 17. It is clear that the obtained periods at applicationof constant operating conditions are non observable. The authorsmodeled the functioning of the drying system based on heat andmass transport within the dried matter, the ambient air and theborder between the two areas, and using the unsteady case. Theefficiency of the drying system was calculated using the followingequation:

� = �W · r

P · 86, 400(3)

where �W is, for the first studied case, the increase in the dailydrying rate when using IR lamps, referenced to purely solar dryingand the increase in the daily drying rate using floor heating systemreferenced to purely solar drying for the second case.

P is the assumed power output of the IR heating for the first

case and for the second case is the average power output of thefloor heating system.

The study must be completed by an economical study toknow the real performances of the proposed drying systems with

ing and after adding an auxiliary source of energy [35].

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upplementary heating sources. Also, in some cases, question ofracticality must be treated.

Seginer et al. [36] have used one of the drying chambers existingn Füssen in Germany. The drying chamber has a concrete floor with0 m wide and 50 m long from the wall with air inlet to the outletall, where fans are located. The installed capacity of the mixing

ans and the ventilation fans is 150 m3 of air per m2 of floor ander hour. In the experiments the mixing fans can give two rates: 0nd 150 m3 m−2 h−1, however the ventilation fans were operatedt several rates of 30, 100 and 140 m3 m−2 h−1. The fans were con-inuously operated at the designated rates. The sludge was mixedy a robot, on a constant daily schedule, for 4 or 6 h per day. Twoethods were used to measure the loss of water; sludge sampling

nd vapour balance. The first method consists on the determinationf the amount of the solids from the volume, bulk density and dryolids content (DSC) of the sludge. On the other hand, sampling theludge every few days to determine its DSC is effectuated. The sec-nd measuring method consists of measuring the humidity ratio ofhe ventilating air at inlet and outlet then multiplying the humid-ty difference by the density of the air and the nominal volumetricischarge of the ventilation fans Qv. The obtained evaporation rate

s expressed:

= �(wout − win) (4)

Four types of prediction models were considered: physical,dditive, multiplicative and neural network model, the multiplica-ive model has shown best results. The most important predictorsf evaporation rate for the conditions under consideration wereolar radiation, outdoor temperature, ventilation rate and dry solidsontent of the sludge.

. Conclusion

Drying is an essential process during the wastewater man-gement, as it reduces the quantity of sludge that could beanipulated. Drying kinetic is one of the important parameters

hat must be known to have a better understand and control ofhe process. The general behaviour of the drying kinetic duringastewater sludge drying contains a short adaptation period to the

pplied drying conditions. Depending on the origin of the sludge, arst principle period known as a constant drying rate period can bebserved. This period is generally followed by one or two falling rateeriods then a short final period where bound water is removed.uring all the process the product has shown shrinkage and crackshenomena.

Using free solar energy for wastewater sludge drying can be ben-fit in point of view of energy consumption and in consequence onhe cost of the drying system. This review has shown that in wholeresented cases, the wastewater sludge is spread in thick layers, inhe floor of a greenhouse chamber, which is the main part of theresented drying systems. Fans and ventilations can be added, inrder to have homogeneous distribution of the inlet air at the sur-ace layers of the product. Also, ventilation is used to evacuate theaturated air and replace it by fresh air. Generally mixing the prod-ct manually or using a robot, once or several times in a day wasffectuated which permits to have harmonized distribution of theried product. Several, materials were added in order to increasehe efficiency of the drying systems, such as heat pump, infraredamps, heating the floor of the greenhouse using solar water heaterr adding rock-bed as thermal energy storage system. According to

he obtained results and the cost of the operation, it was not bene-t, in all cases, to use a second source of energy beside to free solarnergy. The performances of the drying systems change throughhe period of use, the geographical situation of the place where the

[

Energy Reviews 16 (2012) 1061– 1073

experimental work was done, what is more it depends on the originof the wastewater sludge.

Several works have modeled solar drying of the wastewatersludge. It passes commonly through the establishment of heat andmass transfer applied for the air and the product with taking inconsideration the variations of the solar radiations, inlet and outlettemperatures and added materials such as fans, ventilation, auxil-iary sources and storage systems when they are used.

It was confirmed that using solar drying plays an interestingrole for the pathogen reduction of the wastewater sludge [9–11,37].The fecal coliform content decreases until EPA Class B pathogenrequirement and even EPA Class A pathogen requirement, in somecases, such as at the use of a limited amount of lime.

For the most realized drying systems exhaustive modeling stud-ies are needed for having more details about air and producttemperature and humidity, which should give necessary informa-tion for an accurate design of solar drying of wastewater sludgesystem. Also, economical studies concerning the different usedmethods are required for calculation of the cost of the used systemsand their investment feedback.

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